Cysteine Biosynthesis Controls Serratia marcescens Phospholipase Activity

causes health care-associated opportunistic infections that can be difficult to treat due to a high incidence of antibiotic resistance. One of the many secreted proteins of is the PhlA phospholipase enzyme. Genes involved in the production and secretion of PhlA were identified by screening a transposon insertion library for phospholipase-deficient mutants on phosphatidylcholine-containing medium. Mutations were identified in four genes ( , , , and ) that are involved in the flagellum-dependent PhlA secretion pathway. An additional phospholipase-deficient isolate harbored a transposon insertion in the gene encoding a predicted serine -acetyltransferase required for cysteine biosynthesis. The requirement for extracellular phospholipase activity was confirmed using a fluorogenic phospholipase substrate. Phospholipase activity was restored to the mutant by the addition of exogenous l-cysteine or -acetylserine to the culture medium and by genetic complementation. Additionally, transcript levels were decreased 6-fold in bacteria lacking and were restored with added cysteine, indicating a role for cysteine-dependent transcriptional regulation of phospholipase activity. mutants also exhibited a defect in swarming motility that was correlated with reduced levels of and flagellar regulator gene transcription. Together, these findings suggest a model in which cysteine is required for the regulation of both extracellular phospholipase activity and surface motility in is known to secrete multiple extracellular enzymes, but PhlA is unusual in that this protein is thought to be exported by the flagellar transport apparatus. In this study, we demonstrate that both extracellular phospholipase activity and flagellar function are dependent on the cysteine biosynthesis pathway. Furthermore, a disruption of cysteine biosynthesis results in decreased and flagellar gene transcription, which can be restored by supplying bacteria with exogenous cysteine. These results identify a previously unrecognized role for CysE and cysteine in the secretion of phospholipase and in bacterial motility.